1. Field of the Invention
This invention relates to the inspection of overlays on substrates, and, more particularly, to a method for performing a partial inspection likely to identify defective parts.
2. Background Information
In a number of electronic circuit chips, I/O (Input/Output) connections from the various internal circuits are made through ceramic substrates extending along the bottom surface of the chip. Each ceramic substrate has an external surface with an external overlay pattern of conductive pads to which external conductors, such as pins, are attached, for example, by brazing, and an internal overlay pattern of conductive pads to which internal connections from the circuits are made, for example, by reflowing very small solder balls.
The process of manufacturing the ceramic substrate includes the lamination of a number of ceramic layers, each of which carry conductive traces extending among various of the conductive pads. Some of these conductive traces extend between individual pads of the internal and external overlay patterns. After the overlay patterns are placed on the ceramic substrate, the substrate is cured in an oven. Since the curing process causes the substrate to shrink, the overlay pattern is applied in a stretched, or oversized configuration. However, the ceramic substrate, and hence the overlay pattern may shrink at a different rate than that which has been predicted, causing the overlay pattern to be too large or too small, or the shrinkage may be uneven, causing the overlay pattern to be distorted. It is therefore necessary to inspect overlay patterns after the process of curing the substrate.
The conventional method for inspecting overlay patterns on substrates uses a camera to determine the dimensions of the substrates, the distortion of the overlay pattern, the position of this pattern on the substrate, and the feature sizes of the pattern. Since the complexity of the overlay pattern exceeds the resolution of the camera system and/or the memory capacity of an associated computer system if an attempt is made to view the entire overlay pattern at once, the pattern is viewed sequentially in a number of portions according to a co-ordinate system. This method requires that either the substrate being examined or the camera be moved among many positions to acquire the data needed to inspect the substrate. Because of this movement, the inspection process is both complicated, requiring an expensive inspection tool, and slow, requiring typically 90 seconds per part inspected.
What is needed is a way to perform essential inspection procedures without the complexity of moving a camera among sequentially-viewed portions of the overlay pattern, or of moving the part being inspected to present various portions thereof to the camera.